kcsan, kbuild: Add option for barrier instrumentation only

Source files that disable KCSAN via KCSAN_SANITIZE := n, remove all
instrumentation, including explicit barrier instrumentation. With
instrumentation for memory barriers, in few places it is required to
enable just the explicit instrumentation for memory barriers to avoid
false positives.

Providing the Makefile variable KCSAN_INSTRUMENT_BARRIERS_obj.o or
KCSAN_INSTRUMENT_BARRIERS (for all files) set to 'y' only enables the
explicit barrier instrumentation.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

scripts/Makefile.lib

@@ -182,6 +182,11 @@ ifeq ($(CONFIG_KCSAN),y)
 _c_flags += $(if $(patsubst n%,, \
 	$(KCSAN_SANITIZE_$(basetarget).o)$(KCSAN_SANITIZE)y), \
 	$(CFLAGS_KCSAN))
+# Some uninstrumented files provide implied barriers required to avoid false
+# positives: set KCSAN_INSTRUMENT_BARRIERS for barrier instrumentation only.
+_c_flags += $(if $(patsubst n%,, \
+	$(KCSAN_INSTRUMENT_BARRIERS_$(basetarget).o)$(KCSAN_INSTRUMENT_BARRIERS)n), \
+	-D__KCSAN_INSTRUMENT_BARRIERS__)
 endif
 
 # $(srctree)/$(src) for including checkin headers from generated source files